Experimental Astro queued rendering

<section class="stage"><div class="heading">QUEUED RENDER <span class="hint">traverse → render</span></div><div class="queued"><div class="tree-mini"><b>ROOT</b><span>A</span><span>B</span></div><div class="arrow">→</div><div class="queue"><span>ROOT</span><span>A</span><span>B</span></div><div class="arrow">→</div><div class="output-mini">HTML</div></div><div class="caption">Pass 1 orders nodes; pass 2 emits output</div></section>

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